Once again, Robert Mueller's "witch hunt" has caught itself a witch. And this is a big one, as The Washington Post reports:

"President Trump's former campaign chairman Paul Manafort is pleading guilty Friday to two criminal charges under terms of a plea deal that includes his cooperation as a potential witness for special counsel Robert S. Mueller III.

"The decision by Manafort to provide evidence in exchange for leniency on sentencing is a stunning development in the long-running probe into whether any Trump associates may have conspired with Russia to influence the 2016 election.

"Manafort's defenders have long insisted that he would not cooperate with Mueller, and didn't know any incriminating information against the president."

As of this writing President Donald Trump has not tweeted about this development, but when he does I imagine it will be to communicate that 1) he barely knows Manafort, and therefore Manafort's crimes shouldn't reflect on him in any way, and 2) Manafort is just a dirty rat like all the other dirty rats who have betrayed him.

White House press secretary Sarah Sanders quickly released a statement saying, "This had absolutely nothing to do with the President or his victorious 2016 Presidential campaign. It is totally unrelated."

Which is possible. But I'm guessing that pardon is off the table.

As Trump said a few weeks ago, "one of the reasons I respect Paul Manafort so much" is that unlike other Trump aides who felt the law come down on them, Manafort never offered to cooperate with prosecutors. "I know all about flipping, for 30, 40 years I've been watching flippers," Trump said. "Everything is wonderful, and then they get 10 years in jail and they flip on whoever the next highest one is or as high as you can go. It almost ought to be outlawed."

For Manafort, who had already been convicted of eight counts in his first trial and who was facing many more, there was really only one reason not to cooperate: If he could be sure that Trump would pardon him. Counting on that happening would have been a big risk to take. Given how politically dangerous such a pardon would be - it would look to all the world like a cover-up - and given Trump's utter lack of loyalty to anyone below him, a pardon may have been unlikely. So Manafort seems to have calculated that since he couldn't count on Trump coming through for him, his only chance to avoid spending the rest of his days behind bars was to talk.

But about what? That's the question, and as I see it, there are a few possibilities:

- He has information about crimes, but they have nothing to do with the Trump campaign. For years, Manafort was deeply involved in a shady world of Russian oligarchs, possible Russian intelligence operatives, post-Soviet corruption, and money laundering. He may have information that would help prosecutors bring cases against other figures from that world, and perhaps help American intelligence officials understand it better in ways that assist American national security.

- He has information about what happened in the Trump campaign, but that information doesn't touch the president directly. There were multiple people on the Trump campaign who had contacts with Russians during the campaign and who may have been entry points for a Russian attempt to develop a cooperative relationship. Those included Michael Flynn, Donald Trump Jr., Jared Kushner, and Jeff Sessions. Manafort could shed more light on how those Russian contacts came about, what they amounted to, and what kind of cooperation, if any, actually took place.

- He has information about Trump himself. This would obviously be the most explosive possibility. So far we have no direct evidence that Trump was involved in an attempt by his campaign to coordinate with representatives of the Russian government, even though he did publicly implore Russia to hack into Hillary Clinton's emails, which they apparently began doing that very day.

Furthermore, there is a good deal of circumstantial evidence to suggest that Trump knew about the infamous Trump Tower meeting with representatives of the Russian government before it happened, which he has denied. Manafort could shed light on this question, in addition to others that we may not yet have even heard of, since it's a fair bet that Mueller is investigating some matters that have yet to be publicly revealed.

Right now we have no way of knowing which one of those three possibilities is the right one. But if nothing else, we should remind ourselves that Manafort is now the fifth former Trump aide to plead guilty to charges stemming from the Mueller investigation; the others are Michael Flynn, Rick Gates, George Papadopoulos, and Michael Cohen (who pleaded guilty to charges brought by the U.S. Attorney in New York, though that case started with Mueller). Mueller also obtained guilty pleas from two other people (Alex van der Zwaan and Richard Pinedo) and indicted 26 Russians on charges related to the attack on the 2016 election.

All of which is to say that this has been an extremely fruitful special counsel investigation already. And it isn’t done yet.